The Call of Duty: Modern Warfare 3 - Dedicated Server App is only supported for use with the Mac App Store release of Call of Duty: Modern Warfare 3.PLEASE NOTE: The Dedicated Server app is currently only supported on OSX 10.11.6 and 10.12.
1) Launch Modern Warfare 3.
2) From the game launcher, select Download Dedicated Server App.
3) Select Free Download from the Aspyr web page.